This invention relates generally to AC current sensors, and in particular to an improved formable coil for an AC current sensor.
U.S. Pat. No. 5,057,769 discloses an AC current sensor for detecting and measuring alternating current (AC) in a conductor without cutting or breaking the conductor. The AC current sensor comprises a ∪-shaped main coil which defines a measuring recess in which the sensed AC current-carrying conductor is located when a measurement is taken. According to the prior art, it is preferred that the main coil be a long solenoidal coil which is wound with a single layer of windings of wire, and which has an air core. It is also preferred that the main coil be wound so that the mean turn area of wire and the number of turns of wire per unit length are uniform along the entire the length of the main coil in order to reduce sensor errors, and thereby increase measurement accuracy, and to reject the effects of outside current fields.
There are several drawbacks in providing a current sensor in which the main coil is a long solenoidal coil which is wound with a single layer of wire. Refer to the example shown in FIG. 1 of the drawings, wherein a coil having a length L is wound in a single layer of insulated wire with each turn placed against a preceding turn. Any variances in wire diameter or thickness in the insulated coating, or gaps in adjacent turns, result in non-uniformity of the number of turns per unit length, causing sensor errors. Also, when the coil is bent into a ∪ shape in forming the sensor, it is difficult to maintain the uniformity of both the wire""s mean turn area and the number of turns of wire per unit length. If the bends are sharp, there will be a tendency for turns of wire to pop out of place at the bend. If there are kinks in the bend, or if the radius of the bend is not smooth, sensor errors will result. Further, once the single-layer main coil is formed into its final desired shape and placed into position in the sensor, both the shape and position must be maintained in order to prevent sensor errors. Another problem associated with single-layer coils is that the turns will not lay exactly as placed due to the tensile strength of the wire, resulting a in slight unwinding over the length of the coil after it is completed. On a coil wound on a cylindrical mandrel, this is noticeable as a slight expansion in the finished coil diameter; however, on a coil wound on a rectangular or flat mandrel, a definite skew or twist is exhibited over the length of the coil. In the production of current sensors, the forming and positioning of the main coil must be repeatable so that any desired number of current sensors may be manufactured quickly and simply.
In the prior art, an attempt to solve some of these problems involved winding the main coil on a length of straight, flexible plastic tubing prior to bending the main coil into the desired shape. Since the precise length of the main coil is relatively unimportant, it was made as long as possible and the diameter of wire turns made relatively small in order to maintain smooth bend radii and uniformity of mean turn area and number of turns per unit length. The trade-off with this arrangement is that measurement sensitivity is reduced. One coil disclosed in U.S. Pat. No. 5,057,769 was 30 AWG double insulated magnet wire wound in a single layer on a quarter-inch diameter form 16.5 inches long at a rate of 90 turns per inch.
In order to reduce the coil length to under four inches for a hand-held measuring instrument, and to increase measurement sensitivity and accuracy as well as to improve manufacturability of the current sensor, it was desired to provide a main coil of smaller dimensions that could be formed into a desired shape without the constraints of the prior art. The approach taken to solve this problem was to use a finer-gauge wire (smaller diameter) wound in multiple layers to provide the desired number of turns.
An attempt to increase the number of turns to improve sensitivity by winding a second layer over the first layer resulted in several manufacturing problems. The number of turns was difficult to control, and often the turns of the second wrap would fall into gaps created between adjacent turns of the first layer. Moreover, even when a second wrap could be attained, forming the coil into a ∪ shape resulted in an unstable skewing of the coil turns around the arc of the bend. Even with some sort of stiffener, such as plastic tape or a strip of plastic, adhered to the inner side of the coil along its length, problems in producing a stable, formable coil persisted.
In accordance with the present invention, a stable, formable coil exhibiting a high degree of accuracy and high rejection of unwanted or extraneous fields has been attained by winding the turns of the coil in opposite directions in multiple passes, with a selected number of passes in each direction to fill in any gaps and provide a total desired overall number turns. From the coil length and number of turns required, the pitch of the wind, the number of turns per pass, the number of passes required, and the wire gauge can be determined to provide a stable helical-wound coil. A preferred embodiment is wound on a collapsible mandrel to facilitate free removal of the coil from the mandrel. With an equal number of passes in each direction, errors are canceled.
It is therefore one object of the present invention to provide an improved coil for an AC current sensor.
It is another object to provide a sensor coil with improved performance characteristics that is easily formed into a desired shape.
It is a further object to provide an improved coil that is easily fabricated.
It is yet a further object to provide a repeatable manufacturing process for sensor coils using a collapsible winding mandrel.
Other objects, features, and advantages of the present invention will become obvious to those having ordinary skill in the art upon a reading of the following description when taken in conjunction with the accompanying drawings.